1. Field of the Invention
This invention relates to a surface light source capable of varying the angular range for diffusing emitted light and a liquid crystal display apparatus capable of displaying selectively with a wide view angle and a narrow view angle by using such a surface light source.
2. Description of the Related Art
Liquid crystal display apparatus comprises a liquid crystal display device having a screen region formed by arranging a plurality of pixels for controlling transmission of light in the form of a matrix and a surface light source arranged opposite to the viewing side of the liquid crystal display device so as to emit light substantially to all the surface of the liquid crystal display device with substantially uniform intensity. The surface light source includes a plate-shaped transparent member which is a light guide plate, at least at one of the end surfaces thereof being made to operate as light incident end surface, one of the two opposite plate surfaces thereof being made to operate as light emitting surface for emitting light as light enters from the light incident end surface, the other plate surface thereof being made to operate as light reflecting surface for reflecting light entering through the light incident end surface, and one or more than one light emitting elements for emitting light toward the light incident end surface (see, for example, Unexamined Japanese Patent Application KOKAI Publication No. 2000-113706).
Meanwhile, liquid crystal display apparatus are designed to provide a wide view angle and hence a third party can peep into what is being displayed there from a direction inclined relative to the front forward direction (the normal direction of the liquid crystal display device).
Therefore, there has been proposed a liquid crystal display apparatus capable of selectively displaying an image with a wide view angle and a narrow view angle for preventing a third party from peeping in so as to provide an enhanced level of security by arranging a view angle limiting liquid crystal device adapted to reduce the visibility from directions inclined relative to the front forward direction on one of the surfaces of the liquid crystal display device (see, Unexamined Japanese Patent Application KOKAI Publication No. 2004-133334).
A view angle limiting liquid crystal device is formed by arranging electrodes of a predetermined profile on the inner surfaces of a pair of oppositely disposed substrates at positions corresponding to partitioned regions and adapted to align the liquid crystal molecules of the liquid crystal layer filled in each of the partitioned regions between the pair of substrates in a sealed state so as to bring them either into an alignment state that provides a view angle inclined in a direction relative to the normal direction of the liquid crystal display device or into another alignment state that provides a view angle inclined in the direction opposite to the above inclined direction.
The proposed liquid crystal display apparatus can limit the view angle of the displayed image of the liquid crystal display device by reducing the visibility from inclined directions by applying a voltage to the electrodes of the view angle limiting liquid crystal device.
More specifically, the liquid crystal display device provides a wide view angle for the image being displayed when no voltage is applied to the electrodes of the view angle limiting liquid crystal device and hence the view angle limiting liquid crystal device is not operating for displaying an image. On the other hand, when a predetermined voltage is applied to the electrodes of the view angle limiting liquid crystal device, the image being displayed is concealed from side viewers trying to view the image from inclined angles relative to the front forward direction by the partitioned regions of the view angle limiting liquid crystal device where the view angle is inclined in a direction and the partitioned regions of the view angle limiting liquid crystal device where the view angle is inclined in the opposite direction. Thus, the image being displayed cannot be recognized from the view angle which is inclined in a direction and the view angle which is inclined in the opposite direction, and the view angle of the image being displayed is seemingly limited to consequently narrow the view angle of the displayed image.
However, the view angle limiting liquid crystal device has a plurality of partitioned regions produced by dividing the region of the device that corresponds to the display screen of the liquid crystal display device and the liquid crystal molecules of each of the partitioned regions are brought into an alignment state that may be different from the alignment state of other partitioned regions. In other words, the inner surfaces of the pair of substrates have to be subjected to a complex aligning process (rubbing on aligning films) of differentiating the alignment of each of the partitioned regions from that of other partitioned regions. Thus, manufacturing such a liquid crystal device is difficult.